1. Field of the Invention
The present invention relates to a hydraulic control system for an aircraft. In particular the invention relates to a hydraulic control system for determining hydraulic fluid supply pressure of the hydraulic system from load and speed of a hydraulic servo-actuator so as to provide high energy efficiency.
2. Description of Related Art
Generally, a hydraulic system of an aircraft uses a constant hydraulic pressure controlling method.
A hydraulic pump and a pressure controlling method which are used in the hydraulic system for aircraft, can be categorized as the method using a combination of a constant-delivery capacity type pump with a relief valve, and another method using a variable-delivery capacity type pump for keeping the hydraulic pressure constant as described in "Kohkugaku Nyumon (Translated Title: Introduction to Aeronautics) Vol. 2 of 2", Kantosha, pp 104-109.
In addition, it has been proposed a controlling method of a load sensing hydraulic system for varying the hydraulic pressure in accordance with the load being applied.
As the hydraulic pressure controlling method, a load-sensing hydraulic system for controlling the delivery pressure and the flow rate of the pump in accordance with the load pressure is described in "Shiritai Yuatsu Jissaihen (Translated title: Hydraulic Pressure You Should Know, Practical Edition)", Japan Machinist Publishing Company, pp 101-108.
In addition, Japanese Patent Laid-Open Publication No. 60-146799 discloses a hydraulic control system for decreasing energy loss with respect to the hydraulic pressure loss thereof. In the system, the speed and the rotating direction of the hydraulic pump is controlled by an electric motor so that the hydraulic pressure and the flow rate are adjusted in accordance with the load being applied.
However, the related arts with respect to the constant hydraulic pressure control method and the load-sensing hydraulic system have the following drawbacks.
By the constant hydraulic pressure controlling method, the load to an actuator is not always constant. Rather, the load varies with the changes of the aircraft flying speed, attitude and altitude. On the other hand, by the constant hydraulic pressure control method, the setting pressure should be designated in accordance with the maximum value of the load being applied. Thus, when the magnitude of the external load is small, a large part of the supplied pressure becomes an excessive pressure. The excessive pressure results in heating of the hydraulic device, deterioration of fuel efficiency, reliability, and service life, and so forth as a power loss.
Moreover, in the load sensing hydraulic method, when the constitution of the hydraulic system matches the load being applied, a high energy saving effect can be obtained. However the system can be applied only to the hydraulic system in accordance with a meter-in circuit organization having a control valve disposed on the pressure inlet side of an actuator. In addition, when a pressure feedback pipe for transfering a load pressure to a flow-rate compensation mechanism is long, a response disadvantageously becomes low. Thus, when hydraulic systems used in the aircraft or a ship have a meter-out and meter-in circuit for controlling the actuator and the distance between the hydraulic pump and the actuator is very long, this technique is not useful.
Furthermore, according to the method for controlling the speed and the rotational direction of the hydraulic pump, the rotational direction and the speed should be controlled in accordance with the load conditions. Thus, this method cannot be used for the control system of the aircraft or the ship wherein the hydraulic pump is driven by a main engine which constantly rotates in one direction.